Cellular immunity to the hepatitis B surface antigen (HBsAg) and a liver-specific lipoprotein was studied, using the leucocyte migration test, in 38 asymptomatic blood donors found to have HBsAg in the serum. Sensitization to HBsAg was found in 26% and was related to the presence of liver damage, being detected in 47% of those with elevated serum aspartate aminotransferase but in only 13% with normal enzyme levels. The frequency of sensitization to this antigen in those with chronic persistent or chronic aggressive hepatitis on biopsy was also higher than in those with unrelated or minimal changes. The findings using the liver-specific lipoprotein as antigen were similar and there was a correlation between the results obtained with this and the hapatitis B surface antigen. This study supports the hypothesis that a T-lymphocyte response to hepatitis B virus antigen can initiate an autoimmune reaction to antigens such as liver-specific lipoprotein on the hepatocyte surface, and that this reaction may be of importance in the production of chronic liver damage. In the absence of the T-cell response, the autoimmune reaction cannot occur and the virus is able to establish a harmless symbiotic union with the host.